Lecture 6: 555 Timer

April 19, 2023

Introduction to Engineering Electronics

K. A. Connor

1

Lecture 6: 555 Timer

Energy storage,Periodic Waveforms, and

One of the most useful electronic devices

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Examples of Physical Periodic Motion

• Pendulum• Bouncing ball• Vibrating string (stringed instrument)• Circular motion (wheel)• Cantilever beam (tuning fork)

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Other Periodic Phenomena

• Daily cycle of solar energy

• Annual cycle of solar energy

• Daily temperature cycle• Annual temperature

cycle• Monthly bank balance

cycle• Electronic clock pulse

trains• Line voltage and current

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Daily Average TemperatureAlbany-Troy-Schenectady

• Data (blue) covers 1995-2002• Note the sinusoid (pink) fit to the data

-10

0

10

20

30

40

50

60

70

80

90

1

78

15

5

23

2

30

9

38

6

46

3

54

0

61

7

69

4

77

1

84

8

92

5

10

02

10

79

11

56

12

33

13

10

13

87

14

64

15

41

16

18

16

95

17

72

18

49

19

26

20

03

20

80

21

57

22

34

23

11

23

88

24

65

25

42

26

19

26

96

27

73

28

50

Series1

Series2

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Using Matlab to Produce Audio Signal from Daily Average

Temps

0 200 400 600-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8Original data (normalized)

0 200 400 600-0.5

0

0.5Sinusoid fit to data

• Data is normalized to mimic sound• Data is filtered to find fundamental

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Matlab Window

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Periodic Pulse Train from a 555 Timer

• This circuit puts out a steady state train of pulses whose timing is determined by the values of R1, R2 and C1

• The formula has a small error as we will see

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Using Models• Recall that you should use a model that you

understand and/or know how to properly apply

• To use it properly Check for plausibility of predicted values (ballpark

test). Are the values in a reasonable range? Check the rate of changes in the values (checking

derivative or slope of plot). Are the most basic things satisfied?

• Conservation of energy, power, current, etc.

• Developing a qualitative understanding of phenomena now will help later and with simulations.

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Charging a Capacitor

• Capacitor C1 is charged up by current flowing through R1

• As the capacitor charges up, its voltage increases and the current charging it decreases, resulting in the charging rate shown

VV V

R1

1k

U2

TOPEN = 0

12

C1

1uF

U1

TCLOSE = 0

1 2

0

V110V

IV V

R

V

kCAPAC ITO R CAPAC ITO R

1

1

10

1

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msV(U2:1) V(R1:2) V(V1:+)

0V

2V

4V

6V

8V

10V

Capacitor Voltage

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• Capacitor Current

• Capacitor Voltage

• Where the time constant

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msI(R1) I(C1)

0A

2mA

4mA

6mA

8mA

10mA

Capacitor and Resistor Current

Time


0V

2V

4V

6V

8V

10V

Capacitor Voltage

I I eot

V V eo

t

1

RC R C m s1 1 1

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• Note that the voltage rises to a little above 6V in 1ms.

Time


0V

2V

4V

6V

8V

10V

Capacitor Voltage

( ) .1 6321 e

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• There is a good description of capacitor charging and its use in 555 timer circuits at http://www.uoguelph.ca/~antoon/gadgets/555/555.html

http://www.uoguelph.ca/~antoon/gadgets/555/555.html

http://www.uoguelph.ca/~antoon/gadgets/555/555.html

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2 Minute QuizName___________

Section___True or False?

• If C1 < C2, for a fixed charging current, it will take longer to charge C1 than C2

• If R1 < R2, for a fixed charging voltage, it will take longer to charge a given capacitor C through R1 than R2

• When a capacitor C is connected to a battery through a resistor R, the charging current will be a maximum at the moment the connection is made and decays after that.

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555 Timer

• At the beginning of the cycle, C1 is charged through resistors R1 and R2. The charging time constant is

• The voltage reaches (2/3)Vcc in a time

( )R R C1 2 1

0 693 1 2 1. ( )R R C

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555 Timer

• When the voltage on the capacitor reaches (2/3)Vcc, a switch is closed at pin 7 and the capacitor is discharged to (1/3)Vcc, at which time the switch is opened and the cycle starts over

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555 Timer

• The capacitor voltage cycles back and forth between (2/3)Vcc and (1/3)Vcc at times and

1 0 693 1 2 1 . ( )R R C 2 0 693 2 1 . ( )R C

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555 Timer

• The frequency is then given by

Note the error in the figure

fR R C R R C

1

0 693 1 2 2 1

1 44

1 2 2 1. ( )

.

( )

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Inside the 555

• Note the voltage divider inside the 555 made up of 3 equal 5k resistors

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555 Timer

• These figures are from the lab writeup• Each pin has a name (function)• Note the divider and other components inside

NE555

2

5

3

7

6

4 81

TR

CV

Q

DIS

THR

R

VC

CG

ND

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Astable and Monostable Multivibrators

• Astable puts out a continuous sequence of pulses

• Monostable puts out one pulse each time the switch is connected

5V

Ra

C

0.01

uF

LED

NE555

2

5

3

7

6

4 81

TR

CV

Q

DIS

THR

R

VC

CG

ND

Rb

5V

12

1K

0.01

uFC

R

LED

NE555

2

5

3

7

6

4 81

TR

CV

Q

DIS

THR

R

VC

CG

ND

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Astable and Monostable Multivibrators

• What are they good for? Astable: clock, timing signal Monostable: a clean pulse of the

correct height and duration for digital system

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Optical Transmitter Circuit

Astable is used to produce carrier pulses at a frequency we cannot hear (well above 20kHz)

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Optical Receiver Circuit

• Receiver circuit for transmitter on previous slide

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Clapper Circuit

• Signal is detected by microphone• Clap is amplified by 741 op-amp• Ugly clap pulse triggers monostable to

produce clean digital pulse• Counter counts clean pulses to 2 and

triggers relay through the transistor

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555 Timer Applications

• 40 LED bicycle light with 20 LEDs flashing alternately at 4.7Hz

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555 Timer Applications

• 555 timer is used to produce an oscillating signal whose voltage output is increased by the transformer to a dangerous level, producing sparks. DO NOT DO THIS WITHOUT SUPERVISION

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Tank Circuit: A Classical Method Used to Produce an Oscillating Signal

• A Tank Circuit is a combination of a capacitor and an inductor

• Each are energy storage devices

W W LIM L 1

22 W W CVE C

1

22

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Tank Circuit: How Does It Work?

• Charge capacitor to 10V. At this point, all of the energy is in the capacitor.

• Disconnect voltage source and connect capacitor to inductor.

• Charge flows as current through inductor until capacitor voltage goes to zero. Current is then maximum through the inductor and all of the energy is in the inductor.

0

V110V

U2

TCLOSE = 0

1 2U1

TOPEN = 0

1 2

C1

1uF

V

L1

10uH

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Tank Circuit

• The current in the inductor then recharges the capacitor until the cycle repeats.

• The energy oscillates between the capacitor and the inductor.

• Both the voltage and the current are sinusoidal.

0

V110V

U2

TCLOSE = 0

1 2U1

TOPEN = 0

1 2

C1

1uF

V

L1

10uH

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Tank Circuit Voltage and Current

Time

0s 10us 20us 30us 40us 50us 60us 70us 80us 90us 100usV(C1:1)

-10V

0V

10V

SEL>>

Voltage

I(L1)-4.0A

0A

4.0A

Current

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Tank Circuit

• There is a slight decay due to finite wire resistance.

• The frequency is given by (period is about 10ms)

Time

0s 10us 20us 30us 40us 50us 60us 70us 80us 90us 100usV(C1:1)

-10V

0V

10V

SEL>>

Voltage

I(L1)-4.0A

0A

4.0A

Current

fLC

1

2

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Tank Circuit•Tank circuits are the basis of most oscillators. If such a combination is combined with an op-amp, an oscillator that produces a pure tone will result.

•This combination can also be used to power an electromagnet.

•Charge a capacitor

•Connect the capacitor to an electromagnet (inductor). A sinusoidal magnetic field will result.

•The magnetic field can produce a magnetic force on magnetic materials and conductors.

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Tank Circuit Application

• In lab 9 we will be using the circuit from a disposable camera.

• We can also use this type of camera as a power source for an electromagnet.

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Disposable Camera Flash Capacitor Connected to a Small Electromagnet

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Disposable Camera Flash Experiment/Project

• A piece of a paperclip is placed part way into the electromagnet.

• The camera capacitor is charged and then triggered to discharge through the electromagnet (coil).

• The large magnetic field of the coil attracts the paperclip to move inside of the coil.

• The clip passes through the coil, coasting out the other side at high speed when the current is gone.

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Coin Flipper and Can Crusher

• The can crusher device (not presently in operation) crushes a soda can with a magnetic field.

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Can Crusher and Coin Flipper

• This is an animation a student made as a graphics project a few years ago

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Can Crusher and Coin Flipper

• For both the can crusher and coin flipper, the coil fed by the capacitor acts as the primary of a transformer.

• The can or the coin acts as the secondary. • A large current in the primary coil produces an

even larger current in the can or coin (larger by the ratio of the turns in the primary coil)

• The current in the coin or can is such that an electromagnet of the opposite polarity is formed (Lenz’ Law) producing two magnets in close proximity with similar poles facing one another.

• The similar poles dramatically repel one another

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Magnetic Launchers

• Coilguns/Railguns

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Coilguns & Railguns

• Two types of launchers are being developed for a variety of purposes.

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Where Will You See This Material Again?

• Electromagnetic Fields and Forces: Fields and Waves I

• 555 Timers: Many courses including Analog Electronics and Digital Electronics

• Oscillators: Analog electronics• Clocks, etc: Digital Electronics,

Computer Components and Operations, and about half of the ECSE courses.

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Appendix

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Using Conservation Laws to Derive Fundamental Equations

• Energy stored in capacitor plus inductor

• Total energy must be constant, thus

Energy W LI CVTO TAL 1

2

1

22 2

dW

dtL I

d I

d tC V

dV

d tTO TAL 0

1

22

1

22

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Using Conservation Laws

• Simplifying

• This expression will hold if

• Noting that

dW

dtLd I

d tI C

dV

d tVTO TAL L

LC

C 0

V LdI

d tLL I C

dV

d tCC

V VC L I IC L

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• Note that for the tank circuit The same current I flows through both

components The convention is that the current

enters the higher voltage end of each component

I

+

+

VC

VL

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• Experimentally, it was also determined that the current-voltage relationship for a capacitor is

• Experimentally, it was also determined that the current-voltage relationship for an inductor is

I CdV

d tCC

V LdI

d tLL

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• Applying the I-V relationship for a capacitor to the expressions we saw before for charging a capacitor through a resistor

• We see that

I I eot

V V eo

t

1 I C

dV

d tCC

I I e CdV

d tCV eC o

tC

o

t

0 1

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• Simplifying

• Which is satisfied if

• The latter is the relationship for a resistor, so the results work.

I I e CdV

d tCV eC o

tC

o

t

1

RC IV

Roo

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Lecture 6: 555 Timer